Yeah, Ed is reflection is Shaun's failings, but if you look at the housemates as a whole, that's where the brilliance of the setup lies. Ed is the jobless, drunk, shiftless-layabout. Pete is the dedicated, serious worker (also worth noting that he's the only one with a car). Both resent each other for what defines them. Shaun is the one stuck in the middle, and at a moment's notice, he could swing either way and cement his fate. In the end, it feels like Shaun satsifies a happy medium between the two (and in the same flat, natch).
God, I love this movie.